Process to produce microfibrillated/nanofibrillated cellulose by impacts

ABSTRACT

The present invention relates to a process to produce highly microfibrillated cellulose (MFC) and nanofibrillated cellulose (NFC). The invention also refers to a microfibrillated/nanofibrillated cellulose produced according to such process. The microfib-rillated/nanofibrillated cellulose is obtained by subjecting a cellulose fiber in a slurry of cellulose pulp to multiple mechanical impacts. The cycle may be repeated several times. Non-cutting bars disposed in a ring formation is the preferred configuration. At least two rings concentrically arranged facing each other in high rotation transmit the kinetic energy to the fibers to provide the highly defibrillated microfibrillated/nanofibrillated cellulose.

FIELD OF INVENTION

The present invention relates to a process to produce microfibrillated cellulose (MFC) and nanofibrillated cellulose (NFC). The invention also refers to a microfibrillated/nanofibrillated cellulose produced according to such process. The microfibrillated/nanofibrillated cellulose is obtained by subjecting a cellulose fiber in a slurry of cellulose pulp to multiple mechanical impacts. The cycle may be repeated several times.

BACKGROUND

Cellulose is one of the most abundant organic polymers in nature. It is generally synthesized by plants, but it is also produced by some bacteria. Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of β (1→4) linked d-glucose units. Cell walls of the plants attribute their mechanical strength to cellulose. Cellulose owes its structural properties to the fact that it can retain a semi-crystalline state of aggregation even in an aqueous environment, which is unusual for a polysaccharide. In plant cell, it aggregates regularly along the chain, resulting in inter and intra-molecular hydrogen bonds and hydrophobic interactions, and forms fibrous structures called micro fibrils that, in turn, are composed of elementary fibrils or nanofibrils, which are the basic structural units.

Several sources of cellulose have been used to obtain cellulose micro/nanofibers including hardwood, softwood, soybean, cotton, wheat straw, bacterial cellulose, sisal, hemp, sugar bagasse and others.

Microfibrillated/nanofibrillated cellulose are currently manufactured from a number of different cellulosic sources. Wood is the most important industrial source of cellulosic fibers. Obtaining micro/nano fibrillated cellulose from wood is a challenge. Typically, it requires great amount of energy to overcome the extensive and strong inter-fibrillar hydrogen bonds while preserving intramolecular bonds. In other words, the fibrils are processed in such way that micro/nanoscale diameters are achieved but maintaining the long axial lengths to attain high aspect ratio. Among the various extraction processes proposed so far, most are mechanical. For instance, homogenizer, microfluidizer, super-grinder, grinding, refining, cryocrushing, etc. are mechanical methods.

Besides the simple mechanical means to disintegrate cellulose fibers into MFC/NFC, associations with chemical and enzymatic pretreatments can be used. The usage of different enzymes (cellulases, oxygenases, xylanase, etc.) or chemical modifications (TEMPO—oxidation, carboxymethylation, etc.) may be used as pretreatment in order to reduce the energetic cost on the MFC/NFC production.

Specifically, in the case of MFC/NFC production via simple mechanical means, to the cellulosic pulp is applied high intensity shear forces that lead to the individualization of the fibrils. Amongst those mechanical processes, the homogenization is performed under extremely high pressure and is characterized by the great amount of energy required to fibrillate the fibers. In a homogenization process, a cellulose slurry is passed through a very tiny gap between the homogenizing valve and an impact ring, subjecting the fibers to shear and impact forces, which results in cellulose fibrillation. As an alternative for homogenization, the micro fluidization can be used to obtain micro/nanofibrils typically characterized by diameters ranging from 20 to 100 nm and several tens of micrometers in length. The micro fluidization consists in passing the cellulose suspension through a thin chamber with a specific geometry, e.g., a Z- or Y-shape, with an orifice width of 100-400 μm under high pressure, where strong shear forces and impact of the suspension against the channel walls are produced, resulting in cellulose fibrillation. Although producing a high quality MFC/NFC, both processes faces important challenges in order to become economically feasible: great amount of energy to produce, operational issues such as clogging and industrial scalability.

Also, ultra-fine friction grinding is another technique used for the production of MFC/NFC. Supermasscolloider grinder from Masuko Sangyo Co. Ltd., Japan, is one example commonly used. The production of MFC/NFC may be obtained by passing natural fiber suspensions “n” times through the grinder stones. The shear forces generated from the grinder discs are applied to the fibers leading to cell wall delamination and, consequent individualization of the micro/nanofibrils. MFC/NFC are usually obtained with a diameter in the range of 20-90 nm. Alternatively, disc or conical refiners may also be used to produce MFC/NFC throughout a process that includes both mechanical and hydraulic forces to change the fiber characteristics. Typically, pulp is pumped into the refiners and forced to pass between rotating bars located on a stator and a rotor. Therefore, different types of stress forces are applied to the fiber (crushing, bending, pulling and pushing) between the refining bars of the fillings. Shear stresses like rolling and twisting occur in the grooves. Other mechanical processes can be used such as Ultrasonication, Cryocrushing, Ball milling, Extrusion, Aqueous counter and Steam explosion.

However, there is still a need for a process to obtain MFC/NFC, different from those already known in the state of the art and that provides MFC/NFC with a high degree fibril individualization, great features of physical-mechanical properties and, mainly, that is scalable to any size

Thus the present invention provides a microfibrillated/nanofibrillated cellulose without the use of enzymatic or chemical treatments, being environment friendly and avoiding costly or harmful operations, readily applicable to high throughput demands and elevated production. In the present invention, the microfibrillated/nanofibrillated cellulose is obtained by continuous impacts with non-cutting bars. The present invention also provides a method to process cellulose fibers and to further process MFC or NFC fibers.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is a process to produce microfibrillated/nanofibrillated cellulose and the microfibrillated/nanofibrillated cellulose produced according to such process. The highly fibrillated microfibrillated/nanofibrillated cellulose is obtained by subjecting a cellulose fiber in a slurry of cellulose pulp to multiple mechanical impacts. The cycle may be repeated several times. Non-cutting bars disposed in a substantially ring formation of projections is the preferred configuration. At least two rings substantially concentrically arranged facing each other having several bars as projections in high rotation transmit the kinetic energy to the fibers producing the highly defibrillated microfibrillated/nanofibrillated cellulose. The cellulose fibers may be Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers; dry cellulose fibers, never dry cellulose fibers, microfibrillated cellulose fibers (MFC), nanofibrillated cellulose fibers (NFC) or mixtures thereof.

The present invention surprisingly allows the obtaining of a fibrillated cellulose with micro/nanoscale diameters but maintaining the long axial lengths thereof, which results in a micro/nanofibrillated cellulose with a high degree fibril individualization and great features of physical-mechanical properties. The physical-mechanical properties are related with the viscosity, breaking length, tensile index, burst index and elongation of the obtained micro/nanofibrillated cellulose.

A first embodiment of the present invention is a process to produce microfibrillated/nanofibrillated cellulose, which process comprises the steps of:

a) providing a slurry comprising cellulose fibers, b) subjecting the slurry to defibrillation under continuous impacts to produce microfibrillated/nanofibrillated cellulose.

The microfibrillated/nanofibrillated cellulose may be returned as a slurry to step a) to another defibrillation step b). Preferably, the impacts are provided by non-cutting bars, more preferably the non-cutting bars are in a rotor, in a stator or in both, which at least one is rotating. Also, it is provided a microfibrillated/nanofibrillated cellulose produced according to the process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the present process.

FIG. 2 is a drawing representing impact zones and turbulent zones between rotor and stator, as well as the pressure difference regions, according to one embodiment of the present invention.

FIG. 3 is a scanning electron microscopy (SEM) micrograph of a Eucalyptus Kraft cellulose fiber.

FIG. 4 is a scanning electron microscopy (SEM) micrograph of a microfibrillated cellulose produced by traditional disc refiner.

FIG. 5 is a scanning electron microscopy (SEM) micrograph of a microfibrillated cellulose produced by the process of the present invention.

FIG. 6 is a scanning electron microscopy (SEM) micrograph of a microfibrillated cellulose produced by the process of the present invention.

FIG. 7 is a scanning electron microscopy micrograph of a microfibrillated cellulose produced by the process of the present invention.

FIG. 8 is a scanning electron microscopy micrograph of a microfibrillated cellulose produced by the process of the present invention.

FIG. 9 is a scanning electron microscopy micrograph of a microfibrillated cellulose produced by the process of the present invention.

FIG. 10 is a flow curve of a slurry having 0.85% wt. of samples 1-9 of microfibrillated cellulose produced by the process of the present invention.

FIG. 11 illustrates the viscosity profiles of MFC obtained by different processes.

FIGS. 12 a-12 d illustrate the profiles of physical-mechanical properties of MFC obtained by different processes, when MFC is added to a reference cellulosic pulp under the same conditions.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention comprises providing impacts in the cellulose fibers to produce highly fibrillated microfibrillated/nanofibrillated cellulose fibers. The impacts may be provided by any means and are preferably provided by non-cutting bars.

An embodiment of the present invention is a process to produce a highly microfibrillated/nanofibrillated cellulose, which process comprises the steps of:

a) providing a slurry comprising cellulose fibers, b) subjecting the slurry to defibrillation under continuous impacts to produce microfibrillated/nanofibrillated cellulose.

FIG. 2 is a schematic view of the present process having the non-cutting bars disposed in concentrically circles forming rings in a rotor and a stator.

Each of the at least two non-cutting bar rings have an axis defined at its center. The rings are provided with rotating means and rotate. In one embodiment, the rotor ring rotates, and the stator ring is static. In another embodiment, the rotor ring is static, and the stator ring rotates. In a further embodiment, the rotor ring and the stator ring rotate in contrary directions.

The non-cutting bars may be projections in the rotor, the stator or in both. Two non-cutting bars form a bar gap between them, and the non-cutting bars alternate with projection gaps in the ring formed at the rotor, the stator or in both. The gap between two sequenced non-cutting bars in the same ring has at least 1 mm.

When the rotor and the stator with concentrically bar rings are matched facing each other, a ring gap of at least 200 μm is formed between the rings. Applying rotation to the rotor, the stator or both transmits kinetic energy to the bars that impact the fibers disposed through the bar gaps of the rings.

Preferably, the impacts to the cellulose fibers are provided by non-cutting bars disposed in a rotor or stator, preferably projecting from the rotor or stator, preferably from projecting both.

The projecting non-cutting bars are disposed in, or projected from, the rotor, the stator, or both, in a ring configuration, forming a circle or ring in the surface of the rotor, stator, or both. In a ring configuration, the rotor, the stator, or both, when rotating, also rotates the ring formed with the bars, providing a linear speed to the ring. Preferably, the bars are at a linear speed from 20 to 200 m/s, preferably 70 m/s. In one embodiment, the rotor ring rotates at a linear speed of at least 20 m/s, preferably 70 m/s, and the stator ring is static. In another embodiment, the rotor ring is static, and the stator ring rotates at a linear speed from 20 to 200 m/s, preferably 70 m/s. In a further embodiment, the rotor ring and the stator ring rotate in contrary directions, each at a linear speed from 20 to 200 m/s, preferably 70 m/s.

As shown in FIG. 1 , slurry having the fiber, in the form of a cellulose pulp, is fed to the process tank. The slurry consistency may be adjusted to values from 2.0 to 8%, preferably 3.5% to 5%, even more preferably of 4%.

The process of the present invention may also comprise at least one pH modifier added to the slurry during the treatment of the slurry, if modified microfibrillated cellulose is desired. In this case, the slurry is treated before the fibrillation process. The pH of the slurry may be corrected to values from 4.0 to 9.0, preferably, 8.0. If the pH should be corrected to a more basic pH, pH modifiers as ammonia, hydroxides as sodium hydroxide and potassium hydroxide, and others, as sodium hypochlorite, may be used. If the pH should be corrected to a more acidic pH, an acid selected from acetic acid, phosphoric acid, nitric acid, hydrochloric acid and sulfuric acid may be used.

Once the parameters are adjusted, the slurry having the cellulose fibers is subjected to successive cycles throughout the equipment where the fibrillation occurs. The substantially concentric non-cutting bars at the rotor and the non-cutting bars at the stator are disposed to produce a ring gap of at least 0.2 mm between the bars at the rotor and the bars at the rotor, stator, or both are subjected to a high linear speed from 20 to 200 m/s, preferably 70 m/s. The present invention provides that the rotor, stator or both rotor and stator may be rotating or only one of the rotor, the stator may be rotating.

FIG. 2 depicts the non-cutting bars of the rotor and the non-cutting bars of the stator disposition, forming a ring or circle, and the ring gap formed between. The fiber suspension slurry is discharged preferably in the inner zone of the concentrically rings and moves outwardly to the edges due to the rotation of the rotor, the stator, or both. The slurry moves from the inner zone of the stator non-cutting bars ring, reaching the non-cutting bars of the stator, where the fibers in the slurry are subjected to an event of impacts. After receiving the impacts of the stator non-cutting bars, the fibers in the slurry move to a turbulent shearing zone formed between the non-cutting bars of the rotor and the non-cutting bars of the stator. In a continuous outward movement, the fibers reach the non-cutting bars of the rotor, where the fibers are subjected to another event of impacts, producing microfibrillated/nanofibrillated cellulose. Additionally, during the transition from one ring to the subsequent one, fibers are subjected to high pressure changes.

Without being bound by theory, it is believed that the impacts on the fibers, together with the shear turbulence created in the ring gap between the rotor and stator rings with non-cutting bars, and the great pressure drop/increase created by the acceleration and deceleration of the rotor, produce the high degree of cellulose fibrillation. In this sense, the slurry is kept in the impact loop (cycle) for a period that varies depending on the proportion of solids in the slurry. For example, when 200 kg to 500 kg of slurry, at 4% of solids, is used, the cycle of impact varies from 5 to 240 minutes. Due to the heat generation during processing, the suspension may have the temperature controlled between 30 and 100° C.

The impact event defibrillates the fibers and continuously produce microfibrils. The microfibrillated/nanofibrillated cellulose produced may be returned to step a) as a slurry to another defibrillation step b). The process of the present invention may have as many cycles as necessary to produce a highly microfibrillated/nanofibrillated cellulose having a diameter from 0.01 μm to 0.8 μm (10 nm to 800 nm). Preferably, the highly microfibrillated/nanofibrillated cellulose has an average diameter of 0.1 μm (100 nm), determined by scanning electron microscopy. When at 0.85% wt. in water the microfibrillated/nanofibrillated cellulose produced according to the process of the present invention has a dynamic viscosity from 15-1000 mPas·s measured on a rotational rheometer using vane geometry.

The use of impacts for producing microfibrillated cellulose and the use of non-cutting bars for producing microfibrillated cellulose via successive impacts produces a highly fibrillated cellulose having a high aspect ratio.

The fibers capable of producing the microfibrillated/nanofibrillated cellulose of the present invention are cellulose fibers, Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers, dry cellulose fibers, never dry cellulose fibers, microfibrillated cellulose fibers (MFC), nanofibrillated cellulose fibers (NFC) or mixtures thereof.

In one embodiment, the present invention is achieved by a process to produce a highly microfibrillated cellulose, which process comprises the steps of providing a slurry comprising cellulose fibers such as Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers, dry cellulose fibers, never dry cellulose fibers or mixtures thereof. In such embodiment, the process of the present invention will produce highly microfibrillated cellulose by subjecting the slurry of Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers, dry cellulose fibers, never dry cellulose fibers or mixtures thereof to defibrillation under continuous impacts.

In another embodiment, the present invention is achieved by a process to produce a highly nanofibrillated cellulose, which process comprises the steps of providing a slurry comprising cellulose fibers such as Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers, dry cellulose fibers, never dry cellulose fibers or mixtures thereof. In such embodiment, the process of the present invention will produce highly nanofibrillated cellulose by subjecting the slurry of Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers, dry cellulose fibers, never dry cellulose fibers or mixtures thereof to defibrillation under continuous impacts.

In another embodiment, the present invention is achieved by a process to produce a highly microfibrillated cellulose, which process comprises the steps of providing a slurry comprising cellulose fibers such as microfibrillated cellulose fibers (MFC). In such embodiment, the process of the present invention will produce highly microfibrillated cellulose by subjecting the slurry of microfibrillated cellulose fibers (MFC) to further defibrillation under continuous impacts.

In another embodiment, the present invention is achieved by a process to produce a highly nanofibrillated cellulose, which process comprises the steps of providing a slurry comprising cellulose fibers such as microfibrillated cellulose fibers (MFC) or nanofibrillated cellulose fibers (NFC). In such embodiment, the process of the present invention will produce highly nanofibrillated cellulose by subjecting the slurry of microfibrillated cellulose fibers (MFC) or nanofibrillated cellulose fibers (NFC) to defibrillation under continuous impacts.

Accordingly, the slurry may comprise a mixture of MFC and NFC.

The slurry of MFC or NFC, or their combinations, may be previously obtained by processing the cellulose fibers with disc refiners, conical refiners, or combinations thereof. In this sense, processing the fibers is prevalently refining the fiber in order to decrease its dimensions, previously to subjecting the MFC or NFC to the process to produce a highly microfibrillated/nanofibrillated cellulose, object of the present invention.

FIG. 3 exhibits a scanning electron microscopy micrograph of cellulose fibers before the treatment of the present invention, FIG. 4 exhibits a scanning electron microscopy micrograph of microfibrillated cellulose fibers produced by disc refiner technology using ultra high refining level and FIGS. 5-9 are different magnifications of a typical MFC obtained with the present process.

The rheological behavior of the MFC/NFC obtained shows that it has high dynamic viscosity at low shear rate. When the shear rate is increased, the viscosity values decrease, showing the well-known shear thinning behavior of microfibrillated celluloses.

As per evidenced by micrographs of FIGS. 4 and 5 , the MFC/NFC produced by the claimed process visually shows the differences in terms of fibrils individualization and diameter in relation to the MFC/NFC produced by disc refiner technology. As a result, the MFC/NFC produced by impacts provided with non-cutting bars has much more pronounced resistance properties and ability to interact with other materials, as well as enhanced viscosity features.

Besides the visual proof of the differences in terms of fibrils individualization and diameter, FIGS. 10 and 11 show examples of the change in MFC/NFC viscosity, which is a very desired property in many industries such as textile, cosmetics, household, homecare, personal care, food industry, among others.

FIG. 10 depicts typical flow curves (viscosity ratio versus shear rate) of the MFC at 0.85% mass concentration in water, obtained by the present process.

FIG. 11 illustrates the viscosity profiles of MFC obtained by different processes, including the well-known traditional disc refiner and the process of the invention with impacts provided by non-cutting bars.

In FIG. 11 , the data for the MFC obtained with low refining level and with ultra-high refining level show the obtained data in terms of the viscosity of MFC using disc refiners. The difference between them is explained by the applied refining level. By using the ultra-high refining level, it is achieved a refining limit regarding the applied amount of energy and the obtained viscosity data cannot be further increased with the known refining technology with disc refiners.

EXAMPLES

The following examples will better illustrate this invention. The described particular conditions and parameters represent preferred but not limiting embodiments of the invention.

Example 1

Four samples of MFC were obtained by the following different processes:

-   -   Microfibrillated cellulose with low refining level (obtained         with disc refiners);     -   Microfibrillated cellulose with ultra-high refining level         (obtained with disc refiners);     -   Microfibrillated cellulose with impacts provided by non-cutting         bars; and     -   Microfibrillated cellulose with low refining level (obtained         with disc refiners) and impacts provided by non-cutting bars.

The samples of MFC (consistency of 0.85% wt. in water) were tested in order to analyze the viscosity profiles of MFC in terms of the process of obtaining said MFC.

FIG. 11 illustrates the variation of the viscosity (mPa·s) of each sample with the shear rate (s⁻¹). The data obtained for each sample is described in Table 1 below.

TABLE 1 MFC with MFC with MFC with MFC with low low ultra-high impacts refining and Rota- refining refining (non-cutting impacts (non- tional Shear level level bars) cutting bars) Speed rate Viscosity Viscosity Viscosity Viscosity (rpm) (1/s) (mPa · s) (mPa · s) (mPa · s) (mPa · s) 12.5 16.2 150 180.187 226.727 466.870 25 32.4 71.011 103.853 136.633 294.730 50 64.8 40.765 60.304 83.822 175.327 75 97.2 31.132 46.926 68.142 131.293 100 130 26.398 41.233 60.306 108.237 125 162 23.744 37.380 54.869 93.908 150 194 22.189 34.516 51.588 84.292 175 227 21.077 32.297 49.087 77.482 200 259 20.258 30.441 46.763 71.850 300 389 16.833 24.461 38.758 56.023 1500 1940 9.030 10.526 17.001 21.693

The rows for the MFC obtained with low refining level and with ultra-high refining level show the obtained data in terms of the viscosity of MFC using disc refiners. By ultra-high refining level, one can interpret that the refining limit was achieved.

The process herein described leads to a distinct MFC in terms of viscosity values. In fact, the data for the MFC with impacts provided by non-cutting bars of the present invention represents a new viscosity baseline and, consequently mechanical properties, when compared to the viscosity results achieved by the different levels of the existing disc refining technology.

Surprisingly, the most exciting results are achieved when a MFC obtained from a disc refiner technology with low refining level is used as the cellulose fiber in the slurry of step a) of the present invention, that is, before the step of subjecting the slurry to defibrillation under continuous impacts by non-cutting bars to produce microfibrillated/nanofibrillated cellulose. The viscosity data is drastically increased, as described in the row for the MFC obtained from a low refining level technology, which was further subjected to continuous impacts provided by non-cutting bars.

Example 2

Six samples of cellulose pulp were tested to analyze their physical-mechanical properties, as follows:

-   -   Cellulose pulp (no refining)—Control sample;     -   Microfibrillated cellulose with low refining level (obtained         with disc refiners);     -   Microfibrillated cellulose with medium refining level (obtained         with disc refiners);     -   Microfibrillated cellulose with high refining level (obtained         with disc refiners);     -   Microfibrillated cellulose with ultra-high refining level         (obtained with disc refiners); and     -   Microfibrillated cellulose with low refining level (obtained         with disc refiners) and impacts provided by non-cutting bars.

The control sample is a Eucalyptus bleached Kraft pulp with no refining. Four other samples were obtained by the addition of at least 2% wt., preferably 5% wt., of MFC produced by the refining processes already known in the art (with low, medium, high and ultra-high refining levels obtained with disc refiners, respectively) to the control cellulose pulp. The last sample was obtained by the addition of at least 2% wt., preferably 5% wt., of MFC produced by low refining level with disc refiners, which is further subjected to continuous impacts provided by non-cutting bars.

FIGS. 12 a-d illustrate the results achieved for the six samples for physical-mechanical properties of Breaking Length (Km), Tensile Index (Nm/g), Burst Index (KPam²/g) and Elongation (%), respectively.

The results were normalized by the control sample. That is, the results obtained were analyzed in comparison with the results obtained for the control sample (Eucalyptus bleached Kraft pulp with no refining).

The bar graphs illustrate the great results in terms of cellulosic pulp mechanical resistance when MFC is added to it. The first bar is related with the control sample. The following four bars, which are related with samples A to D, respectively, show that disc refining leads to a MFC that provides good improvement in mechanical resistance. However, after a certain level of refining (energy input), there is no big differences in the results, i.e., there is a plateau for the mechanical resistance properties of samples A to D.

The last bar, related with sample E, shows that, when a MFC obtained from disc refiner with low refining level is used as the cellulose fiber in the slurry of step a) of the present invention, that is, before the step of subjecting the slurry to defibrillation under continuous impacts by non-cutting bars, the mechanical properties of the obtained MFC are drastically enhanced. Said enhanced results would not be possible with the traditional refining technologies. 

1. A process to produce a highly microfibrillated/nanofibrillated cellulose, which process comprises the steps of: a) providing a slurry comprising cellulose fibers, b) subjecting the slurry to defibrillation under continuous impacts to produce microfibrillated/nanofibrillated cellulose.
 2. The process according to claim 1 wherein the microfibrillated/nanofibrillated cellulose is returned as a slurry to step a) to another defibrillation step b).
 3. The process according to claim 1 wherein the impacts are provided by non-cutting bars.
 4. The process according to claim 1 wherein the impacts are provided by non-cutting bars in a rotor, in a stator or in both.
 5. The process according to claim 1 wherein the non-cutting bars are projections in the rotor, the stator or in both.
 6. The process according to claim 1 wherein a non-cutting bar alternate with a bar gap in the rotor, the stator or in both.
 7. The process according to claim 1 wherein the bar gap between two non-cutting bars in the same ring is at least 1 mm.
 8. The process according to claim 1 wherein the non-cutting bars have a bar gap in a ring disposition at the rotor, the stator or in both.
 9. The process according to claim 1 wherein one of the rotor, the stator, or both, is rotating.
 10. The process according to claim 9 wherein the rotor ring and the stator ring rotate in contrary directions.
 11. The process according to claim 1 wherein the bars are rotating at a linear speed of at least 20 m/s.
 12. The process according to claim 1 wherein the non-cutting bars at the rotor and the non-cutting bars at the stator are disposed concentrically to produce a ring gap of at least 0.2 mm between the bars at the rotor and the bars at the stator.
 13. The process according to claim 1 wherein the slurry is subjected to continuous impacts under controlled temperature from 30 to 100° C.
 14. The process according to claim 1 wherein the fibers are cellulose fibers, Kraft fibers, bleached cellulose fibers, semi-bleached cellulose fibers, unbleached cellulose fibers; dry cellulose fibers, never dry cellulose fibers or mixtures thereof.
 15. The process according to claim 1 wherein the fibers are microfibrillated cellulose, nanofibrillated cellulose, or mixtures thereof.
 16. The process according to claim 15 wherein the fibers are processed with disc refiners, conical refiners, or combinations thereof.
 17. The process according to claim 1 wherein the slurry comprises a consistency from 2.0 to 8%.
 18. The process according to claim 1 wherein at least one pH modifier is added to the slurry, so that modified microfibrillated/nanofibrillated cellulose is formed.
 19. The process according to claim 18 wherein the pH modifier will produce a slurry having a pH from 4.0 to 9.0.
 20. The process according to claim 18 wherein the pH modifier is selected from the group consisting of ammonia, sodium hydroxide, potassium hydroxide, sodium hypochlorite, acetic acid, phosphoric acid, nitric acid, hydrochloric acid and sulfuric acid.
 21. Microfibrillated/nanofibrillated cellulose produced according to the process of claim
 1. 22. Microfibrillated cellulose produced according to the process of claim 1 having a diameter in from 0.01 μm to 0.8 μm.
 23. Microfibrillated cellulose, according to claim 22, having an average diameter of 0.1 μm, determined by scanning electron microscopy.
 24. Microfibrillated cellulose, according to claim 22, wherein at 0.85% wt. in water comprises a dynamic viscosity from 15-1000 mPas·s.
 25. (canceled) 